1. Field of the Invention
The present invention relates to a multi-leaf collimator that narrows the radiation field emitted to an object and a radiotherapy unit provided with the same.
2. Description of the Related Art
Radiation therapy has been widely used as treatment for affected parts such as cancer and tumors. This radiation therapy is treatment with the destruction of tissue cells in an affected part, inhibition of cell division, or the like by emitting radiation to the affected part. It is important to precisely irradiate an affected part in such radiation therapy. By precisely irradiating an affected part, damage to normal cells around an affected part can be minimized, thereby enabling effective irradiation to an affected part.
In order to ensure precise irradiation, generated radiation is narrowed with a diaphragm part or multi-leaf collimator so that the shape of the radiation field becomes similar to that of the affected part. A multi-leaf collimator comprises a plurality of leaf blocks and a movable mechanism paired with these leaf blocks. The leaf blocks are made of a material such as tungsten that absorbs radiation. By moving the leaf blocks between a radiation source and sites where there is no need to emit radiation, a radiation field is narrowed to a predetermined shape (e.g., refer to Japanese Published Unexamined Application No. 1999-216197).
This movable mechanism is shown in FIG. 1. As shown in FIG. 1, the movable mechanism is provided with a drive motor M, which is a motor with a reducer, and an output gear OG. The leaf blocks are moved with the drive power transmitted from the output gear OG. A plurality of transmission gears TG are arranged between the drive motor M and output gear OG, respectively rotating as they engage with each other, thereby transmitting drive power from the drive motor M to the output gear OG.
In this movable mechanism, location detection part such as an encoder E or potentiometer P are placed on one of the multiple transmission gears TG. Displacement of the leaf blocks is correlated with the amount of rotation of the drive motor M, output gear OG, and the plurality of transmission gears TG. The encoder E and potentiometer P detect the locations of the leaf blocks by detecting the rotation of the transmission gears TG. Furthermore, although the encoder E and potentiometer P have the same function, they both may be arranged for redundancy.
Thus, if a movable mechanism comprises a plurality of gears as above, backlash occurs at each of the gears. With the method of detecting the locations of the leaf blocks based on rotation of one of the multiple gears, errors due to backlash are observed between the detected locations of the leaf blocks and the actual locations thereof.
It is possible to minimize errors caused by backlash, such as by correcting the detected locations of the leaf blocks with the backlash of each gear. However, gears engage with each other physically, so the tops of the teeth become worn over time. As this wear progresses over time, errors caused by backlash will increase. Therefore, even if the locations are corrected with pre-calculated backlash, errors will gradually increase, resulting in decreased accuracy of specifying the locations of the leaf blocks over time.
It is preferable to keep errors in shape between an affected part and a radiation field to within 1 mm around the location of the affected part. Thus, the difference in locations of the leaf blocks must be kept to approximately 0.3 mm. This is based on the ratio of distance between the surface of an installed multi-leaf collimator from a radiation source and the location of an affected part therefrom. Therefore, errors in detected locations caused by backlash affect therapeutic effectiveness to a considerable degree.